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Viewing LazLo's Garden Diary: How to Sterilize Potting Soil
Tuesday, December 15, 2009
Boil it ~ Bake it ~ or ~ Nuke it!
Because so many seedlings are susceptible to fungus (damping off), and because supposedly "sterile" potting media can be contaminated with mold spores, sterlizing the medium before sowing seeds or potting up cuttings can give plants an enhanced chance for success.
Boiling is the method which I prefer - - It seems to be less messy to me and it is far less smelly that the other two methods described below. But YMMV, so I am sharing all three methods of sterilizing potting media.
Boiling: Set pots with your potting medium (soil) in a place where they can drain freely, then soak the medium slowly and carefully with boiling water (as from a tea kettle) until water runs through the drainage holes. Wait for the flow to stop, then repeat the procedure two (2) more times. Allow the potting medium to cool completely before proceding with sowing seeds or potting up cuttings.
Baking in a conventional oven: With a need for a small amount of sterilized potting medium, you can sterilize it in your oven at home. But be aware that potting soil can create a pretty strong smell (and not a good one!) - - So prepare for the smell by keeping your windows open while sterilizing and afterwards to air out your kitchen.
In a deep baking pan, spread the medium evenly, to a maximum depth of four inches. Next, cover tightly with aluminum foil. At the center of the covered baking pan, insert a candy or meat thermometer downward into the medium. Heat the oven to 180°-190°F (82-88°C). Caution: Do not let the temperature of the medium to rise above 200°F (93°C). Higher temperatures can produce plant toxins within the potting medium. Once the medium temperature 180°F (82°C), maintain this temperature for 30 minutes. Then, remove the pan from the oven and allow it to completely cool. Keep the foil cover on top in order to help maintain a sterile environment until you are ready to use the potting medium.
Baking in a microwave oven: For very small amounts of sterilized potting medium that is needed quickly, you can sterilize it in the microwave instead of in an oven. This method will not stink up the house quite as much as the oven method, but only smaller amounts of potting medium can be sterilize at a time using a microwave oven.
Place up to two (2) pounds (907 g) of potting soil into a thick plastic bag such as a freezer bag. Do not use a thin sandwich ("baggie") bag - - It will probably melt in the microwave. Leaving the top of the plastic bag open - for ventilation - place it into the microwave oven. Cook the potting medium on the High setting for two and one-half (2½) minutes. At the sound of the timer bell, immediately remove the bag from the microwave and seal the top. Caution: Do not heat the potting medium for longer than two and one-half minutes else the potting medium will become toxic to your plants. Allow the sealed bag to rest until it is completely cooled. You can store the potting medium in the bag with the top closed.
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Tuesday, December 15, 2009
Re-using Your Old Potting Soil
Have you ever re-used potting soil? I sure have!
Here is how I do it: Put the old potting soil into a 3 gallon pot, then pour a kettle of boiling water through it. Let the water flow stop, repeat with the boiling water 2 more times. When the old potting soil has completely cooled, I mix it, half and half, with new potting soil.
Plants grow very well in this partially recycled medium, and I've never had any root rot problems due to using the old soil.